According to tweeting travelers, many backscatter and millimeter-wave AIT scanning machines at airports are not in use at all, making opting out impossible. We've asked DHS/TSA for comment, but you can help us confirm.

An airline passenger raises her arms as instructed in TSA's new full body scanner at SeaTac Airport Wednesday morning, November 24, 2010. The crowd level was normal, and there were no delays. Peter Haley / Staff photographer

The presumption by some is that the TSA has deactivated the AIT machines in an effort to both increase throughput of travelers through the scanning process and to diffuse protest by the grassroots National Out-Out Day movement.

But as the AP photo shows, it's not always possible for a single passenger to determine if an airport is using the machines at every terminal or not.

We will update this story as we get more information. If you are flying today, it might be useful to simply ask the TSA officers doing the screening if they are using their AIT machines at all.

Update: The TSA Press Secretary Nicholas Kimball responded to my inquiry—"Was there any sort of TSA-wide policy to not use the machines today or is this an airport-by-airport decision?"—with this statement: "No. Nothing to this at all."